Aircraft components are subject to constant degradation such as corrosion and cracking caused by environmental and operational conditions. Although the application of coatings, such as paints, reduces corrosion problems substantially, they typically cannot eliminate them entirely. Furthermore, stress experienced during flight can result in damage which a coating of paint cannot mitigate, such as stress defects and cracking. In order to ensure that aircraft are ready for flight, periodic inspections are necessary.
Inspection of aircraft components traditionally includes visual inspection. When visually inspecting aircraft components, the coating used to protect the components becomes an obstacle because it may hide structural defects or features beneath the coating. It is therefore necessary to strip the component assembly or aircraft in question of its paint before a proper visual inspection can be performed. Afterward, a new coating of paint must be applied. This process results in substantial expense in the form of labor and materials, raises environmental concerns, and requires a great amount of time.
Apart from the inefficiency of visual inspection methods, another problem is that visual inspection is not always effective. While a skillful eye may pick up most human-visible defects with a satisfactory degree of consistency, some defects may be very small or lie under the surface of the component. In many cases these defects will go unnoticed by visual inspection regardless of the skill and experience of the observer.
In addition to visual inspection, active thermography techniques have been proposed for inspection of various components. One such technique utilizes a transient heat source to heat the component, followed by detection of a transient heat signature on the surface of the component to determine the presence of anomalies or defects. However, such techniques require specialized equipment and controls to generate the necessary transient heating, and are inefficient because detection of the transient thermal signature can require a significant amount of time.
U.S. Published Patent Application No. US 2004/0026622 A1 discloses a system for imaging coated substrates which utilizes an infrared (IR) light source. The IR light shines on the object and is reflected to a focal plane array. While such a system may be useful for some applications, an IR light source is required and the incident IR radiation must make two passes through the coating. Furthermore, a portion of the incident radiation may reflect off the surface of the coating, thereby obscuring the image of the underlying substrate.
The present invention has been developed in view of the foregoing.